The present invention relates to the production of laser markable coatings that are durable and have high contrast, without using environmentally suspect materials. The coatings can be used to put identification or tracking marks on objects such as printed circuit boards and other types of products and packaging.
Use of TiO2 as a laser markable pigment in plastic compositions is well known, for example, as disclosed in U.S. Pat. No. 5,091,284. Additives have been combined with TiO2 to increase the contrast by increasing the whiteness of the background, or by improving the dispersion and thereby increase the whiteness, as disclosed in U.S. Pat. Nos. 5,206,280, 5,560,845 and 5,789,466. Similarly, two-layer systems have been used with one contrasting colored layer over another. A laser is used to remove or destroy the upper layer, producing a mark by revealing the underlayer, as disclosed in U.S. Pat. Nos. 4,515,867, 5,111,523, 5,350,792 and 5,599,869. Titanium dioxide has also been coated with a glass forming component prior to its incorporation into the base substrate, as disclosed in U.S. Pat. No. 5,853,955.
Laser marking materials and processes are disclosed in U.S. application Ser. No. 08/951,411 filed Oct. 16, 1997, and U.S. application Ser. No. 08/970,297 filed Nov. 14, 1997, which are incorporated herein by reference.
Conventional laser markable coatings typically use either environmentally undesirable pigmentation-containing materials such as lead and hexavalent chromium to produce durable marks, or use less toxic alternatives that give marks with poor durability. Less toxic pigments are generally unsatisfactory for use in a coating because the mark produced by degrading TiO2 with a laser is of poor durability, and can be easily removed by scrubbing with water or solvent.
The present invention combines a marking pigment that gives a high contrast laser mark with a material that xe2x80x9cfixesxe2x80x9d the mark and improves its durability. The combination of TiO2 or other suitable pigments with a fixing material increases the durability of the mark, rendering it washable and increasing its wearability.
An aspect of the present invention is to provide a laser marking material comprising at least one marking pigment, and at least one fixing material. The fixing material may be bismuth trioxide, antimony oxide, lead oxide, vanadium pentoxide, molybdenum trioxide, alkaline earth silicates and/or alkaline earth aluminosilicates.
Another aspect of the present invention is to provide a laser marking coating comprising liquid carrier and a laser marking material. The laser marking material comprises at least one marking pigment, and at least one fixing material, e.g., bismuth trioxide, antimony oxide, lead oxide, vanadium pentoxide, molybdenum trioxide, alkaline earth silicates and alkaline earth aluminosilicates.
A further aspect of the present invention is to provide a method of marking the surface of a substrate. The method includes the steps of applying a marking material comprising a marking pigment and a fixing material to the surface of the substrate, and irradiating a portion of the marking material with at least one beam to adhere the irradiated marking material on the surface of the substrate and to form a permanent marking thereon comprising the marking pigment and the fixing material.
These and other aspects of the present invention will be more apparent from the following description.